Experience with bulk tungsten test-limiters under high heat loads: melting and melt layer propagation

Sergienko, G.; Базылев, Б.; Hirai, T.; Huber, A.; Kreter, A.; Mertens, Ph.; Nedospasov, A. V.; Philipps, V.; Pospieszczyk, A.; Rubel, M.; Samm, U.; Schweer, B.; Sundelin, Per; Tokar, M. Z.; Wessel, E.

doi:10.1088/0031-8949/2007/t128/016

Cited by 52 publications

(32 citation statements)

References 10 publications

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“…As the effect is strongly increasing at lower magnetic field line to surface angles, one expects a higher saturation level for the sample geometry with a leading edge facing the full parallel plasma flux. This is supported by previous studies of sustained tungsten melting in TEXTOR where the replacement current through a melting tungsten limiter was compared to the Richardson current computed from measured surface temperatures [22]. Along the same line, the previous ASDEX Upgrade transient melt study [12] showed an ongoing increase of the ELM-related current excursions for the leading edge sample in contrast to the observations for the sloped sample.…”

Section: Melt Sample Surface Temperature and Replacement Currentsupporting

confidence: 76%

Investigation of transient melting of tungsten by ELMs in ASDEX Upgrade

et al. 2017

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Repetitive melting of tungsten by power transients originating from edge localized modes (ELMs) has been studied in the tokamak experiment ASDEX Upgrade. Tungsten samples were exposed to H-mode discharges at the outer divertor target plate using the Divertor Manipulator II (DIM-II) system. The exposed sample was designed with an elevated sloped surface inclined against the incident magnetic field to increase the projected parallel power flux to a level were transient melting by ELMs would occur. Sample exposure was controlled by moving the outer strike point (OSP) to the sample location. As extension to previous melt studies in the new experiment both the current flow from the sample to vessel potential and the local surface temperature were measured with sufficient time resolution to resolve individual ELMs. The experiment provided for the first time a direct link of current flow and surface temperature during transient ELM events. This allows to further constrain the MEMOS melt motion code predictions and to improve the validation of its underlying model assumptions. Post exposure ex-situ analysis of the retrieved samples confirms the decreased melt motion observed at shallower magnetic field line to surface angles compared to that at leading edges exposed to the parallel power flux.

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Section: Melt Sample Surface Temperature and Replacement Currentsupporting

confidence: 76%

Investigation of transient melting of tungsten by ELMs in ASDEX Upgrade

et al. 2017

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“…Accurate quantification of the thermionic emission fraction will ultimately require both directly measured surface temperature and comparison to simulations, which also take into account a possible saturation of thermionic emission by space charge formation above the heated surface in presence of a magnetic field [45]. Indications for this effect were previously seen in melt studies at TEXTOR, where the current through an exposed tungsten test limiter probe was measured up to surface melt temperature [46]. Since the molten area can be estimated from the post-exposure melt pattern, a lower bound for the thermionic emission current density can be obtained from the net replacement current, providing a direct qualitative criterion to distinguish transient melting from sustained steady state melting.…”

Section: Measurementsmentioning

confidence: 94%

Experiments on transient melting of tungsten by ELMs in ASDEX Upgrade

et al. 2018

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Repetitive melting of tungsten by power transients originating from edge localized modes (ELMs) has been studied in ASDEX Upgrade. Tungsten samples were exposed to H-mode discharges at the outer divertor target plate using the Divertor Manipulator II (DIM-II) system [1]. Designed as near replicas of the geometries used also in separate experiments on the JET tokamak [2-4], the samples featured a misaligned leading edge and a sloped ridge respectively. Both structures protrude above the default target plate surface thus receiving an increased fraction of the parallel power flux. Transient melting by ELMs was induced by moving the outer strike point to the sample location. The temporal evolution of the measured current flow from the samples to vessel potential confirmed transient melting. Current magnitude and dependency from surface temperature provided strong evidence for thermionic electron emission as main origin of the replacement current driving the melt motion. The different melt patterns observed after exposures at the two sample geometries support the thermionic electron emission model used in the MEMOS melt motion code, which assumes a strong decrease of the thermionic net current at shallow magnetic field to surface angles [5]. Post exposure ex-situ analysis of the retrieved samples show recrystallization of tungsten at the exposed surface areas to a depth of up to several mm. The melt layer transport to less exposed surface areas leads to ratcheting pile up of re-solidified debris with zonal growth extending from the already enlarged grains at the surface.

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“…Although surface melting is observed during disruptions in Alcator C-Mod, a quantitative assessment is not available yet. Controlled experiments on melt layer behavior are mainly performed in TEXTOR using bulk W test limiters [21]. Spectroscopic measurements of the atomic tungsten flux from a hot W surface indicate that no enhancement of atomic tungsten release exceeding physical sputtering and normal thermal sublimation for temperatures below 3700 K occurs under the present experimental conditions.…”

Section: Erosion and Meltingmentioning

confidence: 87%

Experience With High-$Z$ Plasma-Facing Materials and Extrapolation to Future Devices

Neu

2010

IEEE Trans. Plasma Sci.

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The use of refractory metal plasma facing components requires intensive research in all areas, i.e. in plasma wall-interaction, in the physics of the confined plasma, diagnostic, and in material development. Only a few present day divertor tokamaks -mainly Alcator C-Mod and ASDEX Upgrade -gained experience with the refractory metals molybdenum and tungsten, respectively. ASDEX Upgrade was stepwise converted from graphite to tungsten PFCs. In line with this transition a reduction of the deuterium retention by almost a factor of ten has been observed due to the strong suppression of D co-deposition with carbon. The deuterium retained in W is in line with laboratory results in contrast to Alcator CMod, where the D retention in Mo is more than a factor of ten larger than in corresponding laboratory experiments. As expected from the sputtering threshold of Mo and W, negligible erosion by the thermal divertor background plasma is found in these experiments under low temperature divertor conditions. However, erosion by fast particles and intrinsic impurities, which additionally might be accelerated in rectified electrical fields observed during ion cyclotron frequency heating, plays an important role. The Mo and W concentrations in the plasma centre are strongly affected by plasma transport and variations up to a factor of 50 are observed for similar influxes. However, it could be demonstrated that sawteeth and turbulent transport driven by central heating can suppress central accumulation. The inward transport of high-Z ions at the edge can be efficiently reduced by 'flushing' the pedestal region caused by frequent edge instabilities. Extrapolations to ITER and DEMO are difficult since the physics of plasma transport is not yet completely understood, the particle and energy fluxes are orders of magnitude higher and the technical boundary conditions in DEMO strongly differ from those of present day devices.

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Experience with bulk tungsten test-limiters under high heat loads: melting and melt layer propagation

Cited by 52 publications

References 10 publications

Investigation of transient melting of tungsten by ELMs in ASDEX Upgrade

Investigation of transient melting of tungsten by ELMs in ASDEX Upgrade

Experiments on transient melting of tungsten by ELMs in ASDEX Upgrade

Experience With High-$Z$ Plasma-Facing Materials and Extrapolation to Future Devices

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